The present invention relates to a transparent light guide plate that diffuses light from a linear light source in a direction of plane so that uniform illuminating light emerges from a light exit surface, as well as a planar lighting device using the same.
A liquid crystal display device is provided with a backlight unit for irradiating a liquid crystal panel (LCD) with light from its rear side and illuminating the liquid crystal panel. A known backlight unit is one that uses a flat plate of transparent resin as a light guide plate for converting light from a light source into planar light. The backlight unit using such light guide plate usually comprises an illuminating light source, a light guide plate that diffuses light emitted from the light source to irradiate the liquid crystal panel, a prism sheet or diffusion sheet for causing uniform light to be emitted from the light guide plate and other components.
Recent years are seeing demands for reducing the thickness and power consumption of liquid crystal display devices and to meet such demands, light guide plates of various shapes have been proposed. (See, for example, JP 9-304623 A, JP 8-62426 A, and JP 10-133027 A.)
FIG. 15 is a schematic sectional view of the area light source device disclosed in JP 9-304623 A.
The area light source device (backlight unit) shown in FIG. 15 is formed by embedding a fluorescent lamp 102 in a light guide plate 100, then arranging a reflective sheet 104 on a rear surface of the light guide plate 100 and laminating a transmitted light amount correction sheet 106, a light diffuser 108, and a prism sheet 110 on a light exit surface of the light guide plate 100.
The light guide plate 100 has a substantially rectangular shape and is made of a resin into which fine particles functioning to diffuse illuminating light are dispersed. Also, the light guide plate 100 has a flat upper surface, which is used as the light exit surface. Further, grooves 100a each having a U shape in section are formed in the rear surface (surface opposite to the light exit surface) of the light guide plate 100. The fluorescent lamp 102 is fitted into each groove. A light amount correction surface 100b for promoting the emission of illuminating light is formed on the light exit surface of the light guide plate 100, except in portions just above the fluorescent lamps 102.
As mentioned above, JP 9-304623 A describes that, the fine particles are mixed into the resin to form the light guide plate 100 and in addition, the illuminating light is promoted to exit by the light amount correction surface 100b formed on a part or all of the light exit surface except in the portions just above the fluorescent lamps 102, thus making it possible to reduce the total thickness of the plate and the unnatural unevenness in luminance of the light emitted from the light guide plate 100.
For the purpose of obtaining a backlight for a liquid crystal display device that enables reduction in size, weight, and thickness of the liquid crystal display device as well as reduction in cost and power consumption without reducing the illumination amount of the backlight, JP 8-62426 A describes a light guide plate having a rectangular illumination surface, a groove rectangular in section which is gouged out at a central portion with respect to the short sides in parallel with the long sides and in which a light source is inserted, and a rear surface formed such that a plate thickness is gradually reduced from the groove toward both lateral surfaces on the long sides.
In addition, for the purpose of obtaining a backlight unit that enables a thin frame and reduction of the total thickness of a liquid crystal display device and which is bright with high use efficiency, JP 10-133027 A describes a light guide member (light guide plate) having a parabolic shape in section along the width direction of a concave portion where the light source is arranged, with the depth direction being a major axis.
However, planar lighting devices that use the light guide plates described in JP 9-304623 A, JP 8-62426 A and JP 10-133027 A produce an uneven profile of brightness across the light exit surface and, what is more, the utilization of light is so poor that it has been impossible for the light exit surface to emit uniform and bright light.
Under the circumstances, the assignee of the present invention filed JP 2005-234397 A, disclosing a transparent light guide plate. The plate comprises a rectangular light exit surface, a thick portion, thin edge portions, a parallel groove which accommodates a bar-like light source and inclined rear portions. The thick portion is positioned at substantially a central portion of the rectangular light exit surface in parallel with a side of the rectangular light exit surface. The thin edge portions are formed in parallel with the thick portion and the parallel grove is formed substantially at a center of the thick portion in parallel with the side of the rectangular light exit surface. The inclined rear portions are symmetrical with respect to a plane including a central axis of the bar-like light source and perpendicular to the rectangular light exit surface, and whose thickness is reduced from the thick portion toward the thin edge portions in a direction perpendicular to the side to thereby form inclined rear surfaces on both sides of the parallel groove. In the plate, an end portion of the parallel groove is narrowed toward the rectangular light exit surface symmetrically with respect to a center line of the parallel groove perpendicular to the rectangular light exit surface in a sectional shape of the parallel groove in the direction perpendicular to the rectangular light exit surface, in accordance with a ratio of a peak value of illuminance or luminance of emitted light from the bar-like light source to an average value of the illuminance or luminance of the emitted light at second portions corresponding to the inclined rear portions. The bar-like light source is accommodated in the parallel groove at a first portion of the rectangular light exit surface corresponding to the parallel groove. Having this structure, the light guide plate is thin and lightweight and features sufficiently good light utilization (high efficiency in light issuance) to emit more uniform, less uneven and brighter light from a light exit surface.